The field of the disclosure relates generally to turbomachine complexes, and, more specifically, to control systems for operating turbomachine complexes for power generation.
Known turbomachine complexes contain a common shaft to which the rotors of the compressor, the turbine, and the electrical machine, i.e., motor-generator are coupled. Such known turbomachine complexes for electrical power generation are constrained to rotate at the same speed. In known control schemes for such known turbomachine complexes, the electrical machine is coupled to an electrical power source and is run as a motor during startup. In such known control schemes, the power drawn from the power source drops as the torque produced by the turbine increases in order to maintain shaft speed during startup. After the power drawn from the power source drops to zero, the shaft begins to accelerate until it reaches a slightly higher speed, which the control system uses as a cue to begin applying an electrical load to the shaft from the load bank. This action immediately reduces the speed of the turbine, and consequently the speed of the compressor as well, because they are linked together by the common shaft. As the compressor decelerates, the corresponding drop in pressure at the compressor outlet (and turbine inlet) reduces the torque supplied to the shaft by the turbine, further decreasing the shaft speed, until the control system again switches on the motor. This restarts the process, so that a forced, permanent oscillation is oftentimes established, and the system may require manual intervention by the operator to achieve a pure generating mode.
Also, in such known control schemes, as heat is added to the system over time, pressure built up by the compressor leads to the electrical machine generating more power than supplied by the power supply on account of the turbine coming up to speed on the shaft. In such known control schemes, the electrical machine is switched from operating as a motor to operating as a generator in order for an electrical load to absorb excess power from the shaft. During this transitional state of such known turbomachine complexes, oscillations in the system may commence and manual intervention by the operator will be required to reliably achieve the pure generation mode. Further, in such known control schemes for known turbocompressor complexes used for power generation, the aforementioned oscillation is oftentimes especially burdensome during operation of turbomachine heat cycles utilizing lighter weight turbocompressors with less mechanical inertia, including, for example, those developed for carbon dioxide-based cycles. The problematic oscillations oftentimes leads to decreasing compressor speed along with decreasing pressure of the working fluid, thereby requiring the power supply to power the electrical machine as a motor in order to boost the pressure. In such known turbomachine complexes, alternate cycling between the electrical machine operating as a motor and as a generator may prevent reliable generation of electrical power.